This invention relates to identifying sectors in data tracks without the use of sector headers, and to identifying defective sectors in concentric data tracks on a disc and interrupting data transfer to and from such defective sectors. The present invention also is directed to a headerless track sector format.
In computer systems, data storage equipment employ media having tracks, adjacent to which one or more transducing heads is positioned to read data from and write data to the track. One common such storage device is a magnetic disc drive which employs a magnetic medium having plural concentric tracks. In many magnetic disc drives, particularly those employing hard or rigid discs, the disc tracks are divided into sectors, with each sector having a format employing header fields which precede the data field. Ordinarily the data field in each sector has the same number of bytes as every other sector. Hence, the sectors are of substantially equal length.
Tracks at inner radii of the disc have shorter physical lengths than tracks at outer radii of the disc. It is common to employ different recording frequencies in different radial zones of the disc. This technique, called zone bit recording, permits the physical size of the sectors, and hence the bit density, to be substantially the same over the entire disc surface. Consequently, inner tracks contain fewer sectors than outer sectors. Nevertheless, in a hard sectored disc drive, the layout of the tracks is the same for all tracks in a given recording zone, but different between zones. Also, it is common to employ a "runt" sector at the end of each track that contains fewer bytes than data sectors and typically contains no useful information. These "runt" sectors are present only to occupy excess space in the track.
In a typical disc drive, the header provides information to the disc interface controller concerning the logical data which are stored in the corresponding data field. The header also provides information indicating whether there is a known defect in the sector and what type of defect management has been or is to be used to bypass the defect. More particularly, the sector header indicates the existence of a sector defect, so that data in the data field of the sector are ignored or that data are not written into the sector. The sector management instructions indicate that data are to be read from or written into the next successive sector (if not defective) or from or to some alternate track and sector. The alternate track and sector location is usually provided by the microprocessor, as is well known in the art.
In prior hard disc systems, the sector format includes a header having a header field for sector addressing and defect management followed by a cyclic redundancy checking code (which is a limited error detection code) in a CRC field. The data field follows the CRC field and is protected against errors by a powerful error detection and correction code stored in a trailing ECC field. Most disc drives employ specific software and hardware to quickly correct errors detected in the data field by the ECC with minimal latency. However, errors in the header field are not immediately corrected; a minimum of one disc revolution latency is required before an attempt to re-read the header field can be performed. It has been suggested to add software and hardware to correct header field errors with minimal latency, but such hardware must be very fast and therefore large and expensive to correct errors before the data field is read by the read/write head. Inevitably, header error correction hardware adds to the format overhead, since additional code information must be stored on a medium. Also a larger gap would be required between the header and data PLO field. This added format overhead diminishes the capacity and performance of the disc drive.
Disc drives employ servo systems that control the position of the read/write heads relative to the disc. A dedicated servo system employs a separate disc surface containing servo data for the other disc surfaces of the drive. Typically, a disc stack containing a plurality of discs is employed in the drive so that user data may be recorded on several disc surfaces under the control of servo data on one of the disc surfaces. In an embedded servo system, the servo data is written on the same disc surface as the user data. Radial "spokes" appear at regular intervals on the disc and contain the servo data. The region between spokes, called the logical wedge, contains one or several sectors or partial sectors. Often a sector is divided by a spoke, usually in the data field of the sector.
The format efficiency of a disc drive is defined as the number of bytes available to store user data in the data fields divided by the number of bytes in the entire disc surface, and is given by the following relationship: ##EQU1##
The number of bytes in the data field is equal in all sectors which contain user data. The format overhead includes all the bytes other than in the data field, such as the header PLO, synchronization pattern, identification data, CRC, data PLO, ECC, various gaps and the runt sector. The format overhead also includes the spokes in the case of the embedded servo system. A typical data field contains 512 bytes. The format efficiency typically ranges between 80 and 87% for discs formatted for dedicated servo systems, depending on read/write frequencies, types of heads and read channel circuitry and relative size of the error detection and correction codes. The format efficiency typically ranges between 70 and 85% for discs formatted for embedded servo systems.
One difficulty associated with hard disc drives is that as recording frequencies increase and new recording technologies are utilized, the media overhead associated with the headers increases and the format efficiency decreases. There is, accordingly, a growing need for a sector format in which the format overhead is decreased.